The pancreas has an endocrine gland (endocrine cells) and an exocrine gland (exocrine cells). Pancreatic hormones glucagon, insulin, somatostatin, and pancreatic polypeptide are secreted from pancreatic α cells, pancreatic β cells, pancreatic δ cells, and PP cells, respectively, as endocrine cells, while digestive enzymes such as pancreatic lipase, trypsin, elastase, and pancreatic amylase are secreted from exocrine cells.
Diabetes mellitus is broadly classified into 2 types: type I diabetes mellitus (insulin-dependent diabetes mellitus) and type II diabetes mellitus (insulin-independent diabetes mellitus). Of them, type I diabetes mellitus occurs due to impaired insulin secretion resulting from the destruction of pancreatic β cells producing insulin. A method of regenerating patient-derived pancreatic β cells and transplanting the resulting cells, and a method of transplanting pancreatic β cells differentiation-induced from ES cells or iPS cells as well as a method of attenuating diabetes mellitus by the transplantation of pancreatic progenitor cells is known as treatment methods that have been attempted for type I diabetes mellitus in recent years.
In relation to these treatment methods, there are reports about a method for proliferation of endodermal cells, a method for induction of β cells from the obtained endodermal cells (Patent Literature 1), a method for induction of proliferative endodermal cells from human pluripotent stem cells (Non Patent Literature 1), and a method for induction of proliferative foregut endodermal cells from human pluripotent stem cells (Non Patent Literature 2).
There is also a report about a method for proliferating pancreatic endocrine progenitor cells differentiated from pancreatic progenitor cells (Non Patent Literature 3). These pancreatic endocrine progenitor cells, however, are cocultured with mesenchymal cells. Therefore, the resulting pancreatic endocrine progenitor cells presumably do not have satisfactorily high purity. Furthermore, this report makes no mention about a method for proliferation of pancreatic progenitor cells.
There is a report about a method for induction of NKX6.1-positive pancreatic progenitor cells using human ES cells, and the treatment of diabetes mellitus with highly NKX6.1-expressing pancreatic progenitor cells (Non Patent Literature 4). This report, however, makes no mention about a method for proliferation of pancreatic progenitor cells. Also, it has been reported that insulin-producing cells were differentiation-induced from human ES cells or human iPS cells, transplanted to mice, and studied for their application to the treatment of diabetes mellitus. These reports, however, make no mention about a method for proliferation of pancreatic progenitor cells (Non Patent Literatures 6 and 7).
Pancreatic progenitor cells separated from a living body often halt their proliferation during subculture. Thus, the pancreatic progenitor cells are difficult to efficiently proliferate ex vivo. Sui et al. have reported a method for proliferating ES cell-derived pancreatic progenitor cells using DMEM/F12 medium containing B-27® supplement, FGF10, EGF, and SB431542 (TGFβ inhibitor) (Non Patent Literature 5). This method, however, is still inadequate because the proliferation rate is approximately 30 times in 10 weeks and the obtained pancreatic progenitor cells have purity of approximately 50%.